THIS INVENTION relates to a thermodynamic apparatus having a compressor, an expander and heat exchange means connected to the compressor and the expander, and a compressible working medium arranged to flow in the closed cycle. In particular but not limited the apparatus is a closed cycle and/or an indirect fired apparatus and
Closed cycle thermodynamic apparatuses such as engines, heat pumps, coolers and the like have been known for some time. These apparatuses have a number of desirable attributes such as:
no oxidation or carbonation occurs inside apparatus;
unsuitable fuel sources such as coal or nuclear power can be used;
inert gas, such as helium can be used as a working medium inside apparatus;
gases with a higher specific heat can be used within apparatus to increase performance;
pressures up to 20 atmospheres can be used to reduce apparatus size;
no inlet filtration is required and no turbine blade erosion occurs when the expander is a turbine;
a flat specific fuel consumption when operating below full power can be obtained as compressor entry pressure can be modulated for preserving cycle pressure ratio and stator outlet temperature.
However, operating efficiency of these prior art closed cycle apparatuses is limited to about that of open cycle gas engines, even when equipped with a recuperator for recovering some heat from the exhaust gas.
In view of the requirement of heat exchangers in the closed cycle apparatuses and the added expenses and maintenance work associated therewith, at the present time open cycle engines are usually preferred.
Other thermodynamic apparatuses known to the applicant include an expander having a combustion chamber designed for a specific fuel. Usually the fuel is of a high grade kind. The known apparatuses therefore cannot be used with an alternate fuel or energy source.
Consequently there is abundance of low grade resources such as low grade coal, biomass, municipal waste and the like, which are unused and considered waste materials.
These waste materials are normally left in the open and generally looked upon as environmental scars.
An object of the present invention is to provide a closed cycle thermodynamic apparatus which will at least reduce one or more of the above disadvantages.
A further object of the present invention is to provide an indirect fired thermodynamic apparatus which may be adapted to use low grade fuel resources.
In one aspect therefore the present invention resides in a thermodynamic apparatus comprising a compressor, an expander and heat exchange means connecting the compressor and the expander in a closed cycle, and a compressible working medium arranged to flow in the closed cycle. The apparatus further comprises heat recovery means having a heat transfer medium arranged to recover heat by transferring at least certain amount of the heat to and/or from the working medium.
Preferably the heat recovery means is configured so that the temperature of the working medium is within a first predetermined range of temperatures when entering the compressor and/or within a second predetermined range of temperatures when entering the expander.
The apparatus may have a combustion chamber arranged for preheating the heat transfer medium preceding to transferring heat to the working medium.
It is preferred that the heat exchange means includes a first heat exchanger member and a second heat exchanger member, both of which being connected between the compressor and the expander. The first member is arranged downstream to the compressor, and the second member downstream to the expander. The heat transfer medium is arranged to transfer heat to one of said first and second members and to receive heat from the other of the first and second members.
Desirably the heat transfer medium is arranged to travel in a counter flow relationship with said working medium.
In one form the heat transfer medium is arranged to pass through the second member and to receive heat from the working medium, so that the temperature of the working medium is reduced to within the first range of temperatures. The heat transfer medium is then arranged to be heated in the combustion chamber so that at the second member it can transfer heat to the working medium to increase the temperature of the working medium to within the second range of temperatures.
Typically the combustion chamber includes a burner for heating the chamber and the apparatus has fuel preheating means arranged for preheating fuel for the burner. Suitably the preheating means is in the form of a fuel heat exchange member through which the heat transfer medium and the fuel are arranged to pass and in a heat transfer relationship.
Any one of the heat exchange members may have a first flow path for the working medium and a second flow path for the heat transfer medium. Advantageously the first and second flow paths are conduits arranged in the heat exchanger or exchangers. More advantageously the conduits of the first and second flow paths are arranged parallel to each other and have a common wall.
The apparatus of the present invention therefore has a higher thermal efficiency than the prior art closed cycle apparatuses as it recovers heat energy which is otherwise lost. Theoretically the apparatus according to the present invention can covert up to 100% of heat energy to mechanical power. This represents a jump in efficiency as the prior art heat recovery steam generation turbine has a theoretical maximum efficiency of only 60%.
Further, as the combustion and working medium cycles are separate the fuel in the combustion chamber can be given enough time for a complete combustion. This is not possible with conventional turbines. Fuel sources for the apparatus may range from natural gas, petroleum, nuclear energy sources and coal.
The heat transfer medium may be air, coal mine ventilation gases, water and any other suitable fluid.
In another aspect therefore the present invention resides in a thermodynamic apparatus comprising a compressor arranged for compressing a compressible working fluid, an expander for expanding the working fluid, a heat exchange means interconnecting said compressor and said expander for heating the working fluid, and heating means arranged for heating a heat transfer medium and for supplying the heated medium to the heat exchange means for indirectly heating the working fluid while in said exchange means.
The thermodynamic apparatus of this further aspect of the invention may be of open cycle or closed cycle type. Preferably it is a closed cycle type as hereinbefore described.
Preferably the heating means is a combustor arranged for burning a low grade fuel including low grade coal, biomass or municipal waste, and converting said fuel into the heat transfer medium. Alternatively the heating means may be a solar cell arrangement having a conduit through which the heat transfer medium passes and one or more solar cells for collecting solar energy. The one or more solar cells are arranged to supply heat to the transfer medium.
In one form the combustor is a kiln for burning said fuel for conversion into the heat transfer medium. Typically said kiln is a rotary kiln.
Desirably, said fuel is a low grade coal and the kiln is adapted to convert said coal into flue gas being a form of said heat transfer medium and ash which can be formed as a by-product such as closed or open cell spherical nodules for lightweight building materials. Limestone may be added to convert any sulphur dioxide to calcium sulphate. This should at least reduce the amount of low grade coal which poses environmental problems.
If desired methane gas may also be introduced into the kiln for combustion. This is specially advantageous as very low concentration methane from an underground mine can be put into good use while at the same time removing it from the mine.
The heated transfer medium exiting said heat exchange means may be channelled into the compressor for use as the working fluid or a supplement of the working fluid.
The working fluid from said expander may also be channelled into the combustor to join said heat transfer medium.
Advantageously, the apparatus further comprises heat storage means arranged for storing heat transferred from said heat transfer medium and for controllably releasing heat to be transferred to the working fluid. The heat storage means thereby serves to substantially control temperature of the working fluid and may also be used to even out varying temperatures caused by fuel supplies of varying calorific value and quantity.
Said heat storage means may include a phase changeable material which remains at least partially molten at temperatures expected while the apparatus is operated.
The storage means may have a storage tank containing another heat transfer medium in heat transfer relationship with said phase changeable material.
Typically said phase changeable material is NaCl. The said another heat transfer medium can be a Pb/Sn eutectic alloy.
In one form the heat exchange means includes a first heat exchanger arranged for receiving the heat transfer medium from said heating means, and a second heat exchanger arranged for transferring heat to said working fluid, and the heat storage means is connected to said first and second heat exchangers.
A pump may be provided for circulating said another heat transfer medium around the heat exchange means and the heat storage means.
The apparatus advantageously includes a first preheating means for preheating the fuel before entering the combustor.
The apparatus may also include a second preheating means for preheating air and/or other fuel source before entering the combustor.
A preconditioning means may also be provided for precooling the working fluid before entering the compressor.
It is preferred that at least one of said first and second preheating means comprises a heat exchanger which is arranged so that said heat transfer medium, or working fluid is in a heat transfer relationship with said fuel, or air and/or other fuel source.
Said preconditioning means may also comprise a heat exchange arranged so that said working fluid is in a heat exchange relationship with said heat transfer medium.
A pressure control device may be provided for controlling pressure of said working fluid.